Cosmetic or topical compositions typically comprise one or more particulate components, such as, for example, pigments or dyes, fillers, thickeners, sunscreen agents, and the like. Such particulate components are often insoluble in the respective solvent or carrier system and if so remain dispersed or suspended in the cosmetic or topical compositions.
However, whenever there are changes in the pH and temperature in the surrounding environment, the dispersed or suspended particles may agglomerate with one another and precipitate out of the composition. In physical sunscreen preparations, metal oxide particles, such as titanium dioxide (TiO2) or zinc oxide (ZnO) particles are used for their ability to reflect an absorb UV radiation; however, their tendency to agglomerate favors an uneven distribution of the particles when the sunscreen is spread on the skin, resulting in a sun protection factor (SPF) which is much smaller than the expected SPF, with undesired consequences for the safety of the consumer.
Additionally, in such sunscreen compositions, metal oxide particles having a size appropriate to maintain them on the surface of the stratum corneum (i.e., with an apparent diameter larger than about 5 μm) scatter the incident solar radiation such that the topically applied sunscreen confers a white color to the skin. Topically applied micronized particles with an apparent diameter small than 100 nm scatter incident solar radiation and do not confer the undesired white color to the skin. However, concerns have been raised as to whether these particles remain the surface of the stratum corneum or penetrate further.
Irrespective of their size or of clustering, metal oxide particles which absorb solar UV radiation can transfer their energy to surrounding molecular oxygen, thus generating singlet oxygen. Singlet oxygen and other reactive oxygen species (ROS) generated by topically applied metal oxide particles under UV radiation can react with lipids and proteins in the stratum corneum and add to the direct and indirect damage generated by the residual radiation. Encapsulation or coating has been suggested as a way to circumvent, or at least decrease, the generation of ROS by TiO2 particles. As an example, Published patent application Ser. No. 11/534,074, filed Sep. 21, 2006, the disclosure of which is herein incorporated in its entirety, describes hollow, thermoplastic microsphere-entrapped particulates, such as TiO2. The microspheres, which may be of the Expancel type, are treated with a solvent which opens up pre-existing pores in their surfaces so that the particulates dissolved in the solvent may enter the surface pores, replacing a portion of the ambient gas, typically air. The particulates are thus adsorbed into surface pores of the microspheres. The solvents, which are used under ambient conditions and which are described as not strongly polar so as not to dissolve of break the particulates, include ethanol, hydrocarbons, such as hexane or heptanes, esters such as ethyl acetate, and volatile silicones such as cyclomethicone or low molecular weight dimethicone. Strongly polar solvents, such as acetone, DMF, DMS and strong mineral acids or bases are described as not useful. Once the adsorption is completed, the solvent may be evaporated off, resulting in a free-flowing powder. The microspheres, the interiors of which remain hollow, may be further treated by providing a polymeric coating on the surface which, depending upon the intended disposition of the particulates, can either delay or prevent release of the particulates directly onto the skin. There is no change in the size of the microspheres as a result of the treatment provided.
Further, the smaller the particle size, the larger the active surface area, and the more susceptible such particulate components are toward adverse interactions or interference with other ingredients or components in the cosmetic or topical compositions, which may destabilize the cosmetic or topical compositions or reduce the overall performance thereof.
There is therefore a continuing need for treating or modifying the particulate components of cosmetic or topical compositions in order to eliminate or mitigate the above-described drawbacks and improve the overall stability of the compositions without adversely affecting the chemical and physical properties of the particulate components.
There is also a need for improving the chemical and/or physical properties of the particulate components through surface treatment or modification.